A series of N-acylated alpha-amino acids were synthesized and shown to improve the oral delivery of two protein drugs, salmon calcitonin (sCT) and interferon-alpha. Forty-five compounds in this series were tested in vivo in rats and primates. A significant positive correlation was found between the log P of the acylated amino acids and the decrease in serum calcium following oral dosage of sCT in rats. Such a correlation was not found for interferon-alpha. These derivatized amino acids only weakly inhibited the activity of trypsin or leucine aminopeptidase. Histological examinations of rat intestinal tissue after oral dosing of acylated amino acid/protein combinations revealed no detectable pathology.